1. Technical Field
The present disclosure relates to an internal combustion engine having at least one outlet valve per cylinder, which outlet valve can be actuated via a camshaft and a transmission device, a hydraulic valve clearance compensation element being arranged in the transmission device between the camshaft and the outlet valve, and having an engine braking device, having an engine backpressure brake for building up an exhaust gas back pressure and a compression release engine brake, by way of which at least one outlet valve can be held open at least in an engine braking phase, in particular also in a cam base circle phase.
2. Description of Related Art
Laid open specifications EP 2 143 894 A1 and EP 2 143 896 A1 have disclosed internal combustion engines having engine braking devices and valve clearance compensation mechanisms. Here, in each case one hydraulic valve clearance compensation mechanism is arranged in a valve crosshead. Here, the valve clearance compensation mechanism has a piston which adjoins a pressure space, the pressure space being flow-connected via a check valve to a pressure line which has a constant pressure. A relief line emanates from the pressure space, which relief line opens via a controllable relief valve into an oil outlet opening. Furthermore, a hydraulic additional valve control unit of the engine control device is arranged in the valve crosshead, the control pressure space of which additional valve control unit is flow-connected to the pressure space of the controllable relief valve. The control pressure space is flow-connected via an oil duct to a control pressure line on a counterholder, a counterholder making contact via a stop piston with the valve crosshead on a side which faces away from the outlet valves. As a result of the numerous hydraulic pistons and pressure lines which are arranged in the valve crosshead, high machining and manufacturing complexity of the valve crosshead is required, the valve crosshead being weakened structurally and therefore having to be of correspondingly solid design.
The engine braking devices which are described in the cited documents are in each case a mixed form of an engine backpressure brake and a compression release engine brake, which mixed form is also called, in particular, an EVB (“exhaust valve brake”). Here, the hydraulic additional valve control unit is installed on one side into a valve crosshead of the connecting mechanism, which valve crosshead at the same time actuates two outlet valves. The hydraulic additional valve control unit is fed oil by means of the oil circuit of the respective internal combustion engine which is present in any case. In this type of engine braking devices, the use of hydraulic valve clearance compensation devices requires additional measures, in order to avoid uncontrolled pumping up of the valve clearance compensation device during the engine braking mode, which might lead to serious engine damage. In EP 2 143 894 A1 and EP 2 143 896 A1, this takes place by virtue of the fact that the pressure space of the hydraulic valve clearance compensation device is relieved of pressure during the engine braking mode via a controllable relief valve. The arrangement which is known from the prior art with numerous oil bores and hydraulic pistons in the valve crosshead has the disadvantage that the valve crosshead is weakened structurally and therefore has to be of greater dimensions.
Laid open specification DE 10 2012 100 962 A1 describes a possibility of combining a hydraulic valve clearance compensation means with a relief valve and therefore at the same time implementing an engine braking device and a maintenance-free valve train only by way of a hydraulic valve clearance compensation means. The compression release engine brake is therefore formed by way of the hydraulic valve clearance compensation element. In order to avoid the outlet valve being held open in an undesired manner by way of the hydraulic valve clearance compensation means after the engine braking mode has ended, the following components are also required for a braking mode in addition to the customary components of a hydraulic valve clearance compensation means, however: a relief line with a controllable relief valve including a control line, and a hold-down with a setting screw.
The function of this embodiment is similar to the EVB engine braking device which is described in laid open specifications EP 2 143 894 A1 and EP 2 143 896 A1 and can be described as follows: if the exhaust gas throttle valve is closed, the exhaust gas pressure rises in the outlet duct before the compression (bottom dead centre) to such a pronounced extent that the outlet valve is briefly pressed open by way of the pressure wave of an adjacent cylinder. The piston of the hydraulic valve clearance compensation means which is permanently loaded with engine oil pressure prevents renewed closure of the valve. A small stroke remains, as a result of which a part of the compressed air can already flow out of the cylinder during the compression stroke in the engine. After the top dead centre is reached, the said opening is maintained. The pressure on the piston which then moves downwards is reduced substantially, and the braking performance is improved. As a result of the throttling of the exhaust gas, both the upward and the downward movement of the engine piston can be utilized for braking. At the same time, the relief valve is switched in the engine braking mode, which relief valve opens a relief bore to the high pressure space of the hydraulic valve clearance compensation means. The said relief bore is first of all still closed by way of the holddown, however. At the beginning of the injection stroke, the relief bore is opened by way of the rocker arm movement, the oil escapes and relieves the piston. The “extended” piston of the hydraulic valve clearance compensation means can therefore be reset again and can completely close the outlet valve again.
The abovementioned components are therefore still also necessary for a braking mode in this solution, in the form of the relief line with a controllable relief valve including a control line, and a hold-down with a setting screw, in addition to a classic hydraulic valve clearance compensation means.